In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has been developed drastically, and application to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation is possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, it has become possible that a circuit for driving a pixel, which has been conventionally mounted by an external IC chip, is formed over the same substrate as the pixel by using the TFTs.
A poly-crystalline semiconductor film, which is suitable for manufacturing the TFT, is obtained by crystallizing an amorphous semiconductor film. To crystallize the amorphous semiconductor film, a laser annealing method is generally employed in recent years. This is because general thermal annealing requires 600° C. or more while an inexpensive glass substrate is inferior in heat resistance and easy to deform due to the heat. That is to say, the laser annealing method has advantages that the processing time can be shortened drastically compared with another annealing method using radiant heat or conductive heat and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that almost no thermal damage is given to the substrate. Therefore, the laser annealing method is widely used to crystallize the amorphous semiconductor film formed over the glass substrate.
The laser annealing method described herein includes a technique to recrystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, a technique to crystallize an amorphous semiconductor film formed over a substrate, and a technique to anneal a poly-crystalline semiconductor film. In addition, a technique to flatten or modify the surface of a semiconductor substrate or a semiconductor film is also included. As an example of the laser annealing method, a method is given in which a laser beam emitted from a pulsed laser, typically an excimer laser, is shaped into a square spot having a length of several cm on a side or a linear spot having a length of 100 mm or more on an irradiation surface by an optical system, and the irradiation position of the laser beam is moved relative to an irradiation object (see Reference 1: Japanese Patent Document Laid-Open No. H08-088196).
It is noted that the term of linear herein used does not mean a line in a strict sense but means a rectangle having a large aspect ratio (or an oblong). For example, a rectangle having an aspect ratio of 2 or more (preferable in the range of 10 to 10000) is referred to as a line. A laser beam whose spot shape is rectangular (hereinafter referred to as a rectangular beam) on an irradiation surface may be regarded to be included in a linear beam. The laser beam is shaped into a linear spot in order to secure enough energy density to perform sufficient annealing to an irradiation object, and the beam spot may have a rectangular or planar shape as long as sufficient annealing can be performed to the irradiation object.
Since the excimer laser has high output power and a wavelength which is sufficiently absorbed in a silicon film, which is used as a semiconductor film, the excimer laser is usually used in a laser annealing step. However, the excimer laser has a problem in that it becomes more difficult to keep its stability as the laser has higher output power. Therefore, in order to anneal the irradiation surface homogeneously, it is necessary to keep the stability of the laser oscillator high.
As a more specific method for performing homogeneous laser annealing while keeping the stability high, a method is known in which intensity distribution of a laser beam emitted from a laser oscillator is homogenized on an irradiation surface in such a way that the laser beam is divided using a lens array typified by a cylindrical lens array and then the divided beams are combined using a condensing lens. The cylindrical lens array is an optical system in which a plurality of cylindrical lenses are arranged vertically or horizontally. As another example of the lens array, a fly-eye lens is given in which a plurality of spherical lenses are arranged vertically and horizontally.
Thus, the laser beam is divided using such a lens array so that a portion of the laser beam having high intensity distribution is dispersed, and the divided beams are combined using the condensing lens so that the intensity distribution is homogenized on the irradiation surface. However, when the irradiation object has high reflectivity to the laser beam, the laser beam reflected on the irradiation object returns to the laser oscillator as so-called return light. The return light causes adverse effects that the output power and frequency of the laser beam fluctuate and that the rod of the laser oscillator is damaged, which results in unstable oscillation of the laser beam.